Lol let me start off my saying I've had another HTC First that did this too.... I believe I've posted about those too a few months ago. Anyways I bought a new HTC First and here's my story (again).

I was at school (again) and my phone was around 80%. I put it in my pocket and 20 minutes later I pulled it back out and it was dead. Nothing. I'd also like to mention that a week or so before I left it on charge under my pillow and it got REALLY REALLY HOT. It worked fine after that no problems at all until today. Won't charge or turn on. I'd buy a new battery but you can't take the battery out. I'm using the charger it came with too. No other phones do this but my HTC's lol .-.

By the way i tried holding power for 60 seconds+, VOL-/VOL +, any combination and nothing worked. I'd just buy a new battery but nobody sells them >.< Don't want to sent to HTC or AT&T for $200 AGAIN. Thanks guys, hope it gets fixed. Btw: I tried different chargers too lol.